1. Field of the Invention
The present invention relates to a method of controlling recovery in an electric power steering system, and more particularly, to a method of controlling recovery in an electric power steering system which can enhance the recovery control function during recovery after steering when discriminating a recovery state using a steering angle and a steering angular velocity by determining whether or not the steering angular velocity is more than a predetermined value and carrying out recovery control only in the corresponding recovery state.
2. Description of the Related Art
Generally, a vehicle is provided with an electronic control unit (hereinafter, ‘ECU’) for a vehicle that receives an input of an electric signal detected from various kinds of input sensors and outputs a digital control signal which operates various kinds of actuators at the output side.
Meanwhile, there has been developed an EPS system that enables a light and quick steering operation by reducing the actuating force of a driver's steering wheel according to the speed of a vehicle using such an ECU. As shown in FIG. 1, this EPS system includes a sensor unit 1 such as a speed sensor detecting a vehicle speed, a motor position sensor (or a steering angle sensor in case of a vehicle having a steering angle sensor) detecting the relative steering angle of a motor, a torque sensor detecting the torque of a driver steering wheel, etc., an ECU 2 receiving an input of a signal detected from the sensor unit 1 and outputting various kinds of motor control signals and operation signals for reducing the actuating force of the driver's steering wheel and a motor controlling and driving unit 3 controlling the operation of the motor according to the motor control signals and operation signals from the ECU 2 and operating a steering mechanism by a steering assistance force which is caused by a generating power according to driving of the motor.
The thusly constructed EPS system further includes an appliance unit such as a motor or reduction gear in addition to a general hydraulic power steering system, and thus is increased in friction. Accordingly, the characteristics like the degradation of friction feel or stability during steering are shown.
Therefore, a graph of the relation of a steering torque with respect to a steering input angle will be shown, in which the hysteresis becomes more than those of other hydraulic power steering vehicles. In the prior art, a recovery control function was used to compensate for the demerit.
Especially, a recovery control is carried out while seeing a return speed or retention angle of a steering wheel being not held after steering. That is, if a driver let go of the steering wheel, an assist current by which the steering wheel faces to a neutral position is generated using an absolute steering angle signal, thereby making the steering wheel easily returned after steering.
However, in the prior art recovery control method, a recovery state is discriminated by judging that the driver lets go of the steering wheel if the signal of the torque sensor is smaller than a predetermined value. Thus, if the driver slowly releases the steering wheel without completely letting go of the steering wheel, there occurs a problem that a complete recovery control is not applied since a small quantity of a torque exists in the steering wheel.
In other words, as shown in FIG. 2, a recovery gain for a vehicle speed is set (S1), a recovery pattern for a steering angle is set (S2) and then a recovery current is obtained by multiplying the recovery gain and the recovery pattern (S3).
Afterwards, it is determined whether a steering torque is within Deadzone (Deadzone is a range which a torque can not be felt in) (S4).
As the result of the determining of the steps S4, in a case that, the steering wheel is not positioned in a straight traveling state and the steering torque is within Deadzone because of driver's letting go of the steering wheel, an assist current and the recovery current obtained in the step S3 are summed to obtain a motor driving current (S5). The assist current is calculated with the torque sensor signal and the information of the vehicle speed. Finally, the recovery control function is carried out by controlling the operation of the motor according to the motor driving current obtained in the step S5.
However, also in the above-described recovery control method, a recovery state is discriminated based on the steering torque as in the steps S4, and then recovery control is carried out. Subsequently, in case that the driver operates the steering wheel a little in left and right direction during turning of the vehicle, a frequent alternation between a recovery state and a steering state is instantly occurred. Due to this, a discontinuous motor driving current is generated by a recovery control logic, thereby arousing a problem that the driver feels the steering feeling roughly.